Electrical indicating system



u y 1951 e. H. FRITZINGER 2,558,945

ELECTRICAL INDICATING SYSTEM Filed June 20, 1947 3 Sheets-Sheet 1 PLATE M ILLIAMPERES 0 40 30 I20 I60 200 24c- PLATE VOLTS INVENTOfi 3 .2 fll ATTORNEY July,3, 195] FRlTZlNGER 2,558,945

ELECTRICAL INDICATING SYSTEM Filed June 20, 1947 ATTORNEY y 3, 1951 e. H. FRITZINGER 2,558,945

ELECTRICAL INDICATING SYSTEM Filed June 20, 1947 :s sheds-sheet 3 c J! IglfVENTOR ATTORNEY Patented July 3, 1951 UNITED ;:;STATES PATENT OFFICE ELECTRICAL INDICATIN G SYSTEM George H.:Fritzinger, West Orange, N. J.,-assignor to- Thomas A. :Edisom.Incorporated, 'West Orange, N. J., amorpofation of New Jersey Application June 20, 1947; Serial N0.' 7-55;869

'1 -14 Claims. This invention relates tto electrical systems for indicating, measuring, -or controlling (the termmeasuring being hereinafterconsidered as being included within the meaning of the term indicating) according to variations in an electrical quantity (voltage; current, frequency, etc.) an'impedance (capacitypinductance or resistance), or conditions representable in terms of such electrical quantity or impedance.

"This invention hasparticular utility-in connection with liquid level measuring and' control systems, and is herein illustrated and particularly described inconnection with such" systems, but the present invention. has a wide range "of applications and no: unnecessarylimitation to the particular application herein described is intended.

It is well known Lto'vary a capacitance orresistance according to variations in the'levelpr quantity of a liquid, and to'then measure that capacitance or resistance directly in terms of liquid level or quantity. Particularly, it'is: common to effect a capacitance variation with change in level or quantity of electrically non-conductivesliquids, such as gasoline, byplaci-ng a group of-two or more vertically-extending condenser plates in the tank containing the gasoline. Theseplates have open spaces therebetween 'so'thatbelow the level of measurement the dielectric medium between the plates isgasoline and above that level the dielectric medium isair. Since gasoline. has a dielectric constant equal-"approximately to twice that of air, changesin the-level of the gasoline cause proportional'changes in the capacity of the condenser plates. Typically, in present aircraft applications, this capacitance varies from 400 to 800 mmfd. as from empty to full tank conditions.

This invention is particularly 'adaptedto fulfill the needs for a 'stableand reliable electrical system for aircraft, .for measuring such electrical capacity in terms of liquid level or quantity'and for eflecting controlactions in response to'variations of such electrical capacity. The invention is however not peculiar to the'measurement of electrical capacity; butis applicable-as well, as will be hereinafter apparent, to. the measurement of the other electrical quantities and impedances abovementioned.

Broadly, the presentsystemis; of a type known in the art wherein two A.-C.. voltages are obtained from a common source, one of whichis dependent on a quantity ..under. measurement, and are compared to produce atdeflectionof a movableelement according to their ratio.- Preferably, although'not necessarily, the invention contemplates rectifying the two A.-C. voltages and comparing themby aDJ-C. type of instrument-which is itself also known in the art. Certain novel and"important'ieatur-es and objects of thepresentinvention' are:

' (1') To provide an improved measuring and/or control system of thetype mentioned which is inherently independent of the frequency 'ofthe driving source, and which is'adapted to "work efficiently at any frequency of that source within a'range of the orderiof a'few cycles per secend to 'hundreds'of kilocycles perisecond;

2) To provide" such system which is substantially independent ofthe'imagnitude'of the'driving source;

'(3)' To'provide'an eificient such system which does not draw any power from 'the'driving' source or affect" the" magnitude or character of the voltages under "comparison (4) To provide such a system'in'which undesired" variables! such as temperature-induced eifects are substantially'balanced put;

(5) To prov-ide such system in which at least the variable voltage under measurement is transmitted to a measuring or control circuit by a thermionicstage including grid, cathode and plate-elements connected" to operate with feedback either in cath'ode-follower arrangement or with plate-to-grid coupling;

('6) To provide such system wherein both voltages under comparison are fed to measuring or control devices'bythermionic stages connected tooperate with feedback so that the operation of the stages is substantially independent of the plate voltages and tube characteristics;

(7) To provide such system wherein the voltages under comparison are fed to low-impedance measuring or control circuits and are rectified by thermionic stages of the cathode-follower type; and

(8) .To providesuch system wherein cathodefollower stages are arranged sothat the .final indications or control actions are independent of any steady component of plate current.

Furthermore, it..is :an object to provide a cathode-follower stage which is biased tooperate as arectifier without influence from D.-C. plate current.

Still further, it isan object to provide electrical measuring and/or control systems of the character mentioned which aresimple, straightfor-ward; readily amenable .to analysis, easy to adjust and calibrate, dependable, and econmi= cal to manufacture.

These and other object and features of my invention will be apparent from the following description and the appended claims.

In the description of my invention reference is had to the accompanying drawings, of which:

Figure l is a schematic view of a preferred form of electrical indicating system according to myinvention, illustrated for indicating according to variations in an electrical capacity such for example as is used for measurement of gasoline, or

h condenser I2 and the fixed condenser 15. It is to other liquids;

Figure 2 shows a typical plate-current plate voltage family of curves for a triode, together 'j, with a load line, to illustrate better the operation of the cathode-follower stages included in the system of Figure 1;

condensers l and [6 connected in series also across the source l3. One side of the source I3 is preferably corincted tog'roundas'gat l1, and the voltages to be compared aretypic'ally taken from the impedance elements of the respective divider circuits which are connected directly to ground.

Thus the voltages are taken from the variable be noted however that in the variable divider circuit, thevoltage need not be taken from the variable one of the'two impedance elements since the voltage across .the fixed element varies to the,

same extent, but in reverse direction, as does the Figures 3 and 4 are fractional schematic view s illustrating modified input circuits intended for optional substitution for the lefthand portion of the liquid-level system shown in Figure 1;

Figure 5 is a fractional schematic view illustrating a modified cathode follower circuit intended for optional substitution in Figure 1;

Figure 6 is a schematic view illustrating an indicating and/or control system according to my invention wherein the cathode-follower circuits are adapted to pass A.-C. voltages and the rectification is effected by separate devices;

' Figure 6a is a modified substantially equivalent form of the system of Figure 6;

Figures '7 and 711 show modified forms of the systems of Figures 6 and 5 respectively wherein only a single amplifier stage is employed;

' Figure 8 is a fractional view of a ratio-type instrument adapted for control purposes and usable in place of the indicating instruments illustrated in the foregoing figures;

Figure 9 is a fractional schematic view illustrating an input circuit for my invention for measuring frequency;

Figure 10 is a fractional view showing an alternative form of indicating and/or control instrumentfor the present electrical systems;

Figure 11 is a view showing the system generally of Figure 1 modified to use an indicating instrument of the compensated moving-coil type; and

Figure 12 is a view generally of the system of applications wherein it is desired that the final indications be independent of the frequency of that source, both impedance elements of that circuit are of the same typei. e., capacitive, inductive or resistive. The other circuit may not include any impedance elements at all if the whole voltage of the driving source may be taken as a reference voltage. However, if any fraction of the voltage of the driving source is to be used as the reference voltage, the second circuit will include two fixed impedance elements in series to form a fixed voltage-divider circuit. In the particular application herein illustrated the elements of this circuit are also preferably of the same type so that this circuit will not be sensitive to frequency of the source, but these elements need not be of the same type as are those of the variable divider circuit as will hereinafter appear.

In the embodiment shown in Figure 1 the varivoltage-across the variable element. When the variable condenser is of the type adapted to be immersed in a liquid to have'a variable'capacity proportional to the level or quantity of that liquid, it is desirable that one side of this condenser be connected to ground so as to lessen the possibility of shorts developing across the condenser; in liquid-level measuring systems, therefore, the variable condenser I2 is preferably connected' to the ground line I! and the variable voltage is taken directly from that condenser.

In the foregoing description, it will be understood that the term variable as applied to one of the two voltages under comparison refers to variations in that voltage in response to variations in the quantity under measurement. It will be understood that this voltage may also vary from other influences as in response to variations in the magnitude of the voltage of the source l3 but that such other variations are balanced out and are ineffective to influence the final indications-or control action, as the case may be. A

Since the tank condenser l2 has typically in aircraft applications a low capacity of the order of 400 to 800 mmfd, and since the lead connections to this condenser may be quite long and may therefore have a stray capacity of a comparable order of magnitude, this stray capacity is represented by the fixed condenser l8 connected in shunt with the variable condenser i2.

In order to obtain a maximum percentage denser l2, the capacity of the fixed condenser II should be small-i. e., have a high impedance relative to that of the variable condenser. However, when the capacity of the fixed condenser is small relative to that of the variable condenser, the variable condenser captures only a small per centage of the voltage of the driving source. Therefore, as the capacity of the fixed condenser is reduced, a driving source of greater voltage is required in order to maintain the mean varying voltage at a given value. For best conditions of balance it may be desired that the fixed condenser l I have a capacity equal to the mean value of the condensers l2 and I8. While a wide latitude in choice of the capacities of the condensers l5 and I6 is permitted, for best conditions of balance it may be desirable that the capacities of these condensers be the same as that of the condenser II.

The voltages under comparison are fed by leads l9 and 20 to grids Zla and 22a of respective electron discharge devices 2l and 22. Each of these devices has also a plate and cathode designated by the number of the device with the suffix letters b and'c respectively. These devices may, for

example, be: the vtriode sections of :a 6SN7' tube:

Thewplatesxiofthese devices are connectedsbyia or 'anysuitably compensated instrument adapted" to producerdeflections of a movable element sub stanti'a'lly according to the ratio of two currents; Intany case such instrument will have two coils". difi'erently energizable to produce thedeflectionsr Ofthe strictly ratio-type instruments, there may:

for example housed the ratiometerdescribed in.

Kelly Patent No. 2,362,562," issucdNovember l4, 1944; Aninstrument of this type is 'diagram-' maticallyshown'in Figure l as comprisingtwon field coilsi26and 2'l'.positioned atangles to'each" other :about a central axis 25a, a permanent magl net 28 pivoted at the axis 25a, a pointer 29 carriediwith the per-m'anentmagnet-and a suitablycalibrated" scale 29a" (fractionally shown) with I which the. pointer registers. The operation of this instrument is such thatwhen only one of the two coils 'is energized the magnet aligns.v itself with tha't coil, and when both coils-are energized Y the-magnet takes an intermediate position be tween the coils inproportion to the relative energizations of the: coils.-

The coils of the instrument'25 are connected in a bridge type circuit ac which is adapted to cause i the" current in one coil to decrease to a minimum as- -the'current in the other coil is increased to a maximum, and viceversa. Such'a bridgetype circuit is formed lathe-present instance by connecting the two coils'in series between the cathodes Zlc and 220, and by connecting resistors-3! BZ an'd 33 from the cathodes and from the junction libetween'the coils to a'cornmon junction 35.- Tliisbridge network of coils and resistors forms the principal load for each electron-device 2| and22$this load being that which appears -effectiviely between the respective cathodes and the In the" system current cutoff by returning the grids through grid leak-resistors 35 and-8'?- to groundand by holding the cathode above ground potential by connecting the junction 35 We bleeder circuit comprising resistors 38'and 39 'con-nectedin seriesacross the-plate supply battery 24; Since the bias so produced has always a fixed relation to the plateyoltage, plate current cutoif is maintained: irrespective of variations in the plate voltage. I

The gridleak. resistors have high resistances relative to the impedances across which'the volt-T ages under comparison are: derived. Since the grid-cathode circuits of the electron discharge devices present no load on the voltages -under comparisonin that the 'gridsare always negative relative to the cathodes, as'willhereinafter" appearit is evident that the measuring: cr-controlling apparatus herein provided presents sub"- stantially no'load on the voltage or element undenmeasurement to influence'or aiiect that voltageor element.

The operation of .a cathode-follower stage; is expressed ivory-simply bythe formulauw M mu+l where i is the discharge or cathode .currentpmuur is the amplification factor of the tub'e c is the: dynamic voltage impressed on the grid; Rp is'the' dynamic plate resistance of the tube, 'a,l'1d:R1?.iS-'. the dynamic load impedance. By analogydzofthei conventional amplifying: stage, the propagation factor of a cathode-follower stage is always? somewhat-less than 1, being mu/(mu+1)j but; the effective dynamic plate resistancecf the-tube is very-low, .being R (mu+l) which is onlyape? proximately 650 ohms for one'section of a BSN'Z? tube :abovereferred to. Thus; thecathodefol-x lower is essentially an impedance transformingt device which produces very little loss in ;voltage:. Because of thischaracteristic of the;catho.deeie follower stage, it is admirably suitedtothe-iap-e: plication -of liquid-level measurement because of the typically highimpedance of the immersed. detectingcondenser and the relatively low "immedancez-of the-usual measuring instruments" Since .the 'cathode follower is a voltage.-degenerae tive' one it is characterized; so longiasfthaload'r resistanceis high relative to the effective dynamic plate resistance of the tube, by high-"stabilityrin r response to variations-in the .plate supply. voltage:-

and .or" the tubecharacteristics. Howeverpnote. withstandingthat-the' cathode follower is a de generative oneon a vo1tage.'basis, it has :a high; power amplification factor when thestage is fed;- from a high im-pedance source. 'Thisis because; the cathode-deflower stage has 'a relatively low dynamic plate resistance.-

By reference-to the graphoFFigure 2 there" may be'readily visualized the operation of each"- cathode-follower stagewhen' the tube is biased to plate current cutoff. T This graph shows a family. of the plate-current"plate-voltage curves fOrJa I 6J5 .tube- -theequivalent of one section of a GSN? tube'and: includes a load line f0rc4000 ohms .drawn from a point 0 at.120 v. and 8 v. 'gridbiason the plate voltage axis, this loadof4000 ohms."- being a typical one 'for each' cathode-follower" stage. as is hereinafter apparent During-ieach' negative half cycle of the A.-C.'voltage.-applied1' tothe grid of each cathode-follower stage xthe grid is swung ever more negative and ino" plate currentflows. During eachpositive half cycle,i-:- there .is an excursion up. the -load line 'but' to an: extent which'is only a fraction ofthejrvoltage-g applied to the grid because of the grid-followingactionof the-cathode; lForexample, an:excur'-'" sion'up the load line to zero grid bias T'WlH'CaUSB a peakplate'current of 8.5 magand cause the cathode'potential to rise'by at v. (0085x4000); To produce this zero grid bias the voltageaon the gridmust be raised by the 34 v. of thez-cathode plus the initialx8 v. bias, orby 'a total-of 42 'v.] These alternate half-cycle excursions up the load line cause pulses of-ll-C. current to :flow fro'm' each cathode through the bridge network to the: junction 35 and thence through the resistor 39 'of the :ble'eder circuit to ground. The average :value of this current from the. cathodei22c is fix'edas for any given value of the A.-C2.'source l3, but the averagevalue of the current from the oath ode' 2 lo varies as the condenser l2 varies"; Uporr'u; properlyadjusting'the bridge resistors 31,32 and; 33; a predetermined maximum current irom the cathode i 2.1 0, corresponding vto a minimumnzaluezw of the .-condenser-v 2 will cause :the:currentiy-thesa:

coil 21 to be zero and the current in the coil 26 to be amaximum so asto produce a reading at one extremity of the scale 2911'." Likewise, a predetermined minimum current from the cathode 220, corresponding to the maximum value of the condenser l2, will cause the current in the coil 21 to be a maximum and the current in the coil 26 to be a minimum to cause the pointer 29 to read at the other extremity of the scale. That the currents in the coils will so vary as the potential of the cathode 2Iic is varied while that of the;cathode 220 is fixed is evident from the fact that'the potential of the junction 34 between the coils varies always in the same direction as that of the cathode 210 but to a lesser degree, causing therefore the potential of the cathode 2ic to gradually approach that of the junction 34 as the potential of the cathode falls and causing the potential of the junction 34 to gradually approach that of the cathode 220 as the potential of the cathode 2 lo rises.

It is apparent from an inspection of the graph of Figure 2 that it is desirable to adjust the load resistance and the magnitude of the voltage applied to the grids of the cathode-follower stages so that the half-cycle excursions up the load line will be nearly to the plate-current plate-voltage curve for zero grid bias, for then the operation is principally in the region where the tube characteristics are essentially linear and the ratio of the average plate current of the cathode-follower stages, for a given value of the condenser I2, is essentially constant within a wide range of variation of voltage of the source l3. In this respect it is interesting to note that a reduction in the peak voltage applied to the grids should be ac companied by a reduction in the load resistance, forthen the cathode does not follow the grid so closely and the operation will extend into the linear portion of the tube characteristics. Of course, to minimize the effect of the curved portion of the tube characteristics, there should be used tubes having sharp plate current cutoff.

The minimum required plate voltage for the cathode-follower stages, for any given value of A.-C.' voltage applied their grids, is that which just prevents the grid bias from being overcome. Obviously, as the A.-C. grid voltage is decreased, the plate voltage may be decreased. For example, it is found that upon applying about 25 v. peak to the grid of a 6J5 tube having a cathode load of 4000 ohms, the minimum required plate voltage is'approximately 70 v., but if about 42 v. peak are applied to the grid the minimum required plate voltage is approximately 120 v. as hereinbefore mentioned. If the operation is well into the linear portion of the tube characteristics; variations in the plate voltage above the minimum required value have substantially no influenc on the indications of the meter 25.

Because of the balancing-out character of the voltage-divider circuits, the frequency and, magnitude of the source l3 as well as temperatureinduced variations of the elements in these circuits do not tend to produce any error. Also, the

.final indications are substantially independent of variations in the tube characteristics and the plate supply voltage because of the inherent stability ofthe cathode-follower circuit. A feature of the system shown in Figure 1 which is instrumental particularly in ruling out varying tube characteristics as a source of error is in biasing the tubes to plate current cutoff, for there is no static space-discharge current to influence the final indications. The only space-discharge ages under comparison. These space 'currents' depend only on the relatively stable dynamic characteristics of the tubes.

Althoughthe present system is basically independent of the frequency of the source l3, it is found that the driving torque produced in the meter 25 decreases as the frequency of the source I3 increases. It is believed this is due to the fact that the inductance of the meter coils increases with frequency. This objectionable result is however readily overcome simply by placing condensers 40 across the meter coils. These condensers produce the result of maintaining a substantially steady voltage across the coils and of providing a steady and substantially greater flow of current through the coils, all other con-- ditions being the same. The values of these condensers are not critical since they may be selected Within a wide range.

By way of example, the following components and typical values have been used very satisfactorily in the system shown in Figure 1: A.-C. voltage of source I3, 50 v. R. M. S. maximum; capacity variation of condensers I2 and IS in parallel, 600 to '1000 mmfd; capacity of condensers l I, I5 and I6, 800 mmfd. each; resistances of grid leaks 3B and 31, .4 megohm; space-discharge devices 2| and 22, 1 6SN7 tube; resistances of coils 26 and 21, 1000 ohms each; resistor 3|, 8500 ohms; resistor 32, 6500 ohms; resistor 33, 7800 ohms; resistor 38, 10,000' ohms; resistor 39, approximately 800 ohms; battery 24, v. minimum; and condensers 40, .2 mid. each. With the above values, there is obtained a maximum current through each meter coil of approximately If the operation of the cathode-follower stages is confined principally to the curved portion of the plate-current plate-voltage characteristics, the indications of the meter 25 will vary somewhat with changes in the A.-C. voltage of the source I3 and also with changes in the plate voltage. However, the variation in the indications caused by a change in the voltage I3 is opposite to the variation caused by a change in the plate voltage; in fact, it is found that if these two voltages are maintained in fixed proportion to each other they may be varied through wide ranges Without substantially influencing the meter indications at all. Typically the voltage I3 is obtained from an oscillator which is powered from the same source as that which provides the plate voltage, and the oscillator voltage will then tend to vary in fixed relationship with the plate voltage. However, some change in this relationship may occur in practice from variations in the characteristics of the oscillator tubes, change of oscillator tubes, temperature-variable effects, etc.

If it is desired to hold this relationship fixed, a rectifier may be connected across the oscillator source [3 and be biased in fixed relation to the plate voltage soas to clip off the peaks of the positive half cycles of the oscillator voltage. As

shown dottedly in Figure 1, such clipping may bethe system that the operation will be principally within the linear portion ofthe tube characteristics, as above described, in which case this clipsystem.

ping circuit is--not--ordinarily necessary. *"Ihe maximum voltageof "the positive half cycles-of the oscillator source 1 3 which this clipping-circuit will pass is-determined by thebias voltage on the diode obtained-from the bleeder circuit, provided of course the resistance of the portion of the bleeder circuit from which this bias voltage is obtained is low relative to the effectiveinternal resistance of-the-source l3.- Typically; the diode is biased to clip the normal voltagesupply of theolo- -=be reduced. 'lhus=,-the average values of --the space-discharge"currents-of the two dev-ices 2l and 22 will -\-'ary--according-to the value'of the alternating-currentvoltages impressed on "the oscillator by an-amount equal at leastto the maximum variationeffected' in the oscillator voltage independently of the variation thereof induced by .changes in the voltage of the power supply for the oscillator.

' In Figure 3 there is shown a-modified input circuit for the present measuring systemswherein the fixed voltage circuit," corresponding-to the fixed voltage-divider circuitJA of Figure 1, is

driving'source E This-simplified circuit: may be employed when it ispermissible to impress the whole voltage of the-source [-3 on the grid 22a of the electron discharge device 22. As before, the

2m and 22a is independent of the frequency: and

magnitude ofthe source .13.

In Figure 4 there is .shown another modified form of input circuit for the presentmeasuring In this-embodiment the voltage takencBO from the variable divider-circuit l0 is fed to the grid 2 Ha through a linkcircuit 4| which. isinductively coupledto-the condenser I2 by a transwformer 42 and to the grid.2la by a transformer 43. The advantage of using suchlink circuit is 35 that upon the transformerv 42 having aestepdown ratio and the .transformerlii having a corresponding step-upratio the link circuit will have a low impedance and low stray capacitance. Thus, notwithstanding that the condenser l2 may have a high reactance under the particular operating conditions, itmay be coupled throughlong distances to the grid Zla without encountering .any. substantial loss duetostray -capacity ofthe coupling circuit-or without requiring any special low-capacity shielding. of that circuit.

.Also in Figure 4, thefixed.voltage-divider-cir- .cuit,referred.to as Ma, is shown as seriallycomprising two resistances. and 45in .place' of the condensers l5 and lfito illustrate the flexibilityin type of impedance elements which may :-be employed. Since. both..elements in. thew-fixed divider circuitJdasare-of thesame type. it is apparent that the eratioof the voltagesafied -to the grids 21a and. 22a, is still independent of the frequency ,andmagnitude of ,the .voltage source I 3.

In Figure 5 there is shown a difierent typeof cathode-follower circuit for the. present-measuring system. .This cathode-follower circuit may 6 be used with any of the input circuits hereinbefore described,.whereforethose .circuits need not be here again shown. In this cathode-follower circuit the gridleak resistors36. and E3! are connected from-the grids directly to the respective cathodes, and blocking condenserszdfi are connected in the grid circuits Where necessary; also, the junction :.35.of the bridge .30 is connected directly to ground. As a result, the

:ure 2. When an A.-C. voltage is impressed on the grids,.-the grids/gopositive relative to the 7;

'Figure 1. rectifiers as shown; there-may be used full-wave rectifiersyin which-case the'blocking condensers -55 and resistors 56 are not required.

acteristics.

" cathode and drawcurrent during the positive half cycles. 1 However; as the grids tend itG'fgO positive their impedance from ground f-alls and "theycapture less voltage from theinput circuits;

as a resu1t,-during= the positive hal-fcycles the grid voltages remain. substantially fixed. During-thenegative half cycles of the-=-impressed A.-C. -voltage,-there--are excursions down the load-line irom-the point P causing the' average value 7 of the a static space-discharge current to respective grids. In the cathodecircuits of these devices there is-the bridge 30-including the'c'cils 26 and2l as shown in Figure 1. I he variation .in the effective cathode currents just mentioned "will cause the currents in the -c'oils 26-and-2'I to simply a direct connection of the lead 2tl..to' the -20- "scale-29a according-to the ratioof the impressed vary so-as to deflect -tT.. .e'.poin-ter-29 across the A.C.. voltages as hereinbeiore described. In

this case, however, the readings are dependent on the static' discharge currents of' the-cathoderatio of the voltages-impressed on thetwogrids n25 --better driving torque in the instrument 2 5-"-but -may= be detrimental where a-h-ighdegree of accuracy is --requi-red since thisstatic *discharge follower tubes.= This is beneficial in securing a current,--being dependent on the tube characteristics, mayintroduce a source of error.

-'-In Figure 6' thereis shown a-m0d 1fied system of the character cabove describedwherein the cathode-follower stages are biased-tdpass A.C. voltages without: clipping or -rectiiying those voltages, andsepar-ate devices are employed-for effecting the rectification if des-ired. '-''The'input circuit maybe any of those -hereinbefore described, it bein 'shown'ior-example ascomprise ing the variable voltage-divider circuit I 0- and the fixed divider circuit [4a connected-across the common-source l3. 1 The -v oltages from these circuitsarefed tothe grids of the -two-spaeedischarge devices 2 I and 22. Serially connected between the cathodes and ground of these devices are respective load-resistors and Stand bias -resistors ''52 v and 53, with the latter being connectednearer -to a the cathodes." shunting these bias resistors-are condensers-t aand connected from the respective grids to the lowerpotential sides of these bias resistors --are=*the grid-leak resistors 36 "and 31. i 'Thus the' grids are biased to avoltage intermediate a-zero-bias and that- Which produces plate current cutoff As so biased,the cathode-followerstages will pass A.-C-. voltages *without rectification or clipping.

Connected across each load-resistor 'ifland-5 I is a blockingcondenser 55-andresiston 56=inseries; and connected across each-resistor -ifi is a rectifier 5'! I feeding into the bridge 30 shown in 'Ofcourse-instead ofusing half-wave In this systemof- Figure 6,- the operationof the cathode-follower stages -may "be confined wholly to 'the linear portion of the tube char- -As a 1 result; the operation' of -t-he cathode-follower stages is substantially independent of variations inthe voltage of'the source 13' above any mean-value notexceeding amaximum limit, and is independent also" of variations of the plate supply voltage above the required -minimum-limit." However; this circuit-requires a higher-plate voltage to: produce a givemdr iving ,58 shunted by respective condensers 59.

plates are connected through load resistors 60 cathode-follower type, as shown. half portion (above the ground line H) of this torque inthe instrument 25 than "do the rectifying cathode-follower circuits hereinbefore described.

The present invention comprehends, as a substantial equivalent, the use in Figure 6 of a conventional amplifying stage having a plate-togrid feedback in place of the cathode-follower stages as above shown. Such modified embodiment is indicated in Figure 60.. Here the two grids are returned to ground through the grid leak resistors 36 and 31 and the cathodes are returned to ground through bias resistors 51 and The and BI to a battery 62, and across these load resistors are connected the blocking condensers 55, resistors 56, rectifiers 51 etc., as shown in Figure v6. A feedback circuit serially including a blocking condenser 63 and a resistance 64 is connected from each plate to the respective grid.

By properly selecting the values of the resistors 64 in the feedback circuits, the effective propagation factors of each amplifier stage can be .made very low, in which case the stability of the stage will be enhanced as with respect to varia- 'tions in plate voltage and tube characteristics to approximate the stability and dependability of the cathode-follower stage. Also, such conventional amplifying stage with degenerative feedback, like the cathode follower stage, has very low dynamic plateresistance to enable eflicient power transfer from the amplifier stage to the output meter.

It will be understood that in either of the systems shown in Figures 6 and 6a, each rectifier 51 and respective resistor 56 may be eliminated so as to feed A.-C. voltage to the bridge 30, and

that in such case the D.-C. type of instrument 25 may be replaced by a suitable A.-C. type of instrument.

In Figure 7 there is shown a modified form of the system of Figure 6 employing a single amplifying stage with feedback, preferably of the The upper system is the same as the corresponding portion of the system of Figure 6 and has the same reference characters. However, in the'lower half portion, there are eliminated the thermionic cathode-follower stage 22 and its associated elements 31, 53, 54, 55 and 56, and the lead 25 from the fixed divider circuit la is connected directly to the rectifier 51. This elimination of the second cathode-follower stage is generally permissible, because the thermionic tube 2 I, when connected in cathode-follower arrangement, has the ability to transmit the voltages applied to it .without any substantial variation caused by norpower amplification and a high ratio of 'im-' pedance transformation between the high-impedance voltage-divider circuit l0 and the relatively low-impedancemeasuring or control circuit 30.

As a further illustration of my invention there is shownin Figure'ja a modified form of the system of Figure 5 wherein there isemployed a single cathode-follower stage. Here again substantially the upper half portion of this modified system is the same as the corresponding portion of the system of Figure 5. The thermionic tube 22, which receives the voltage of the fixed divider circuit I4 is however here eliminated. Instead, the source 13 is stepped up in voltage by a transformer H, rectified by a rectifier 12, filtered by a filter generally referred to as 13 and fed to. a bleeder circuit 14 and, by a lead 15, to the plate 2 lb of the thermionic tube 2 I. From the bleeder circuit there is tappedoif a voltage of suitable value which is fed by a lead 16 to a point of the bridge 30 corresponding to that which, in Figure 5, is connected to the cathode of the second thermionic tube 22. It will be observed that so long as the operation of the thermionic stage 21 is substantially independent of its plate supply voltage, the ratio of the two rectified voltages which are fed to the bridge circuit 35 is dependent only on the variable condenser l2, the same as in the system of Figure 5. For the same reasons as ar pointed out above in connection with the system of Figure '7, the cathode-follower stage 2| remains a necessary component of the system.

This invention comprehends that in any of the foregoing indicating systems the instrument 25 may be converted to a control instrument, or relay, by providing the movable pointer with a contact 65 and providing in the path of this movable contact one or more semistationary contacts 66 as indicated in Figure 8. The foregoing systems may thus be converted to control suitable apparatus for fueling or defueling tanks in case the systems are operated from a tank condenser l2 as above described, or may of course otherwise be used in'a wide range of control applications.

In Figure 9, there is shown purely by way of example a type of input circuit for the present system which may well adapt the system for the measuring of frequency. This input circuit comprises one voltage-divider circuit that serially includes two impedance elements 6! of the same type-for example resistorsso as to be independent of frequency; also, the input circuit comprises a second voltage-divider circuit that is sensitive to frequency, including for example a resistance 68 and condenser 69 in series. Both of these voltage-divider circuits are, as before, con- 7 nected across a common source of A.-C. voltage 10 the frequency of which'is under measurement. As the frequency of this source varies, for example, ,the ratio of the voltages taken from the two divider circuits by the leads ll) and 20 will vary and may be indicated by the systems hereinabove described, it being understood however that this ratio is independent of the magnitude of the source 10.

The present electrical systems are not limited to the particular instrument 25 and bridge circuit 30 above described but may employ any suitable instrument for producing a deflection in response or according to the variation in the ratio of the voltages under comparison. In Figure 10, for example, I show an alternative such instrument and circuit. This instrument, referred to generally as H, has 'a rotor 28 and pointer 29 pivoted at 25a, the same as the instrument 25. However, in this instrument each of the field coils is provided in two sections, there being one coil 18 having sections 18a and 18th and a coil 19 'having sections 19:; and. 19b; The coil sections 18a and 19b are serially included in a circuit connected fromxthe cathode 2| 0 to the junction 213 L14 5,:and the coil sections; 19a; and 'IBb-areseniafly zero;v to a;maximumjnyresponse. to variationsiin in u i a circuit 8 onn t rm;.th ztheresistance. undermeasurement; .The current cathode 22c tothe junction 35, this junctionbeing @inuthe compensating. coil,..on .the other hand,

. connected for example to thebleeder circuit ;,varies, only with change. in the. voltaeeofi-the .3833 as in Figure 1.- The sections of. each coil v5 source. Upon poling the. compensating. coil. so I8 and I9 are poled in the circuits, 8!] and 8] so ,th'atits electromagnetic field. opposes that ofthe ;as to magnetically oppose each other; also the (permanent magnet, the; effective magnetic, field sections of each coil are placed in close physical ,across the .poles; 9. I-. isreducedin response to.. an rclationship with one another so that Whenthe increase in-the voltage source to compensate for ampere turns in; the sections are equal there- 10:-)th.e increase of currentinthe moving. coil in'resultant field is effectively neutralized.= A preespouse-to that voltage; increase, .andvicc versa. ferred-form of each coil 18 and I9 is one of a {While I do-herein illustrate my inventionin conaftwin-filament type wherein two wires are --nection with this simplified short-scaleiormwof wound side by side to; form coil-sections having -coxnpensated moving-coi1.instr.ument, it ,will .be equal turns and resistance. :ls understood thatmy invention is applicable ,as From the foregoingdesoription it is understood well, to improved long-scale (approximately???) that the potential of the cathode 2Ic isvaried in moving-coil instrumentsgof this compensated response to variation of the quantity under meastype such as the. improved long-scale instrument -uren1ent and that the potential of the cathode describedin the pendinirsapplication of Jacob .L. .220 is relatively fixed. If the potential. of the 20 Zar; Serial No. 676,613,,filedJune'14,.1946135hd cathode 2Ic is varied say 20% above and below having a commonassignee-with.the presentap- "that-of the coil 220, the coil sections with the .plioation.

suffix letter b should likewise have effectively The systemshown in Figure llh-asabridgecir- 20% less ampere turns than the-respective coil ..cuit,39a corresponding to the;bridgecircuit fltof sections with the sufiix letter a for equal currents ;;Figure .1 but difiering fromthe latter in that the in the circuits 83 and 8|. This is accomplished ycompensatingcoil; 99 ;of-th e above, describeddnby placing adjustable resistors 82 and83, across strument is connected in theplace of, the resistor ,the coil sections 181) and 19b. The resistor 82 is 133 and in that aresistorllill is.,.c.on,nected inthe adjusted so thatwhenthe voltage of the cathode place ofqthe field, coil,:2l.; -,Also, in placeof.,the .2Ic is a maximum the resultant field of the 0011 130 field coil 26 there is connected a moving.coil:.94.

I9 is substantially zero and so that when the volt- .An. additional change in the. system of .Figure age of the cathode 2Ic is a minimum the reill over that of Figure lliesineliminating the sultant field of the coil I8 is substantially zero. sec o nd thermionic stage including the tube 22,

Under these conditions it is apparent that when ;a nd in connecting a rectifier Illlin its placeasln the effective field of one coil is substantially zero the embodiments of Figure 7 and 7a hereinbefore that of the other coil is a maximum. Thus, as -des cribed. The rectifier; IIJI is usedin place; of the voltage of the cathode 2Ic varies between tithetubejlbecause the compensatingcoil-remaximum and minimum values, the rotor 28 will quiresconsiderable electrical powertypically;of .be deflected, as in the instrument 25, to swing the order of .5 W. and asimple rectifier is better the pointer 29 across the full length of the scalesco .able to supply ;this,po-wer than is a;thermionic 29a. When both the coils and the resistors 82 ,amplifierstage. Since this power for. the comand. 83 have the same temperature coefficient of z pensating coil is to be obtained from the, oscil- .electrical conductivity, say that of copper, the -lator source I3; the resistors and-.45 of the fixed currents in the circuits 80 and 8| vary alike with voltage dividercircuit Ma will typically have low change in temperature, and temperature effects valuesv or may desirabl be eliminated asshown are therefore balanced out. -in Figure 3 whenever the full volt ge .of...the In Figure 11 the system of Figure 1 is revised source I3 may be applied to the rectifier, Ifll. :;'Io to include a DArsonval type of electrical instruincrease the flow of current through the coils 94 ment having a compensating coilwhich, when the sand 99 as is hereinbefore described, the same are instrument is connectedv in an ohmmeter type of .shuntedby respective condensers I02 and I03.

circuit, is adapted to compensate for fluctuations Since the potential at the junction. I04 of the in the source of potential used to operate the bridge 30a does not vary with change in thedeohmmeter. A simple ,form of short-scaleinstrutecting condenser. I2 but does change with :the ment 'ofthis type is described in the Harrison voltage ofthe source I3, and the potential, ofthe Patent No. 1,695,424 issued ,Decemberl8, l928. other corresponding junction of the bridge 30a, This instrument comprises a magnet. having the cathode 2Ic, varies both with change in the field poles 9| with confronting arcuate pole faces condenser I2 and the voltage of the s0urcer I3, the "9.2 between which is centered a soft iron. cylin- ..currents in the two coils 94 and 99 vary alikein -drical core' 93. Surrounding this core is a. movresponse to changes in the voltage of the source I3 ing coil 94 pivoted at the central axis of the core,. but. the current in the moving coil varies only and carried with .the moving coil is a pointer 95 with change in the condenser I2. Since the oscilwhich registers with a scale. 96. As sofar. delator source I3 produces like current .changesin scribed, this is a regular DArsonval type of .inboth coils, variations inthe voltage of thi source strument. .To compensate such instr-umentfor are ruled out as asource of error. It may be fluctuations in the voltage supply. when the in- (35 noted that changes in the voltage ofthebattery strument is used in an ohmmeter circuit, the 24 will tend to produce a-relative variation in-the poles of the magnet'QB may be bridged by a soft currents in-the two coils-94 and 99 butthat such iron bar 91, which may have air gaps 98 between :relative vchange will be very small and will proit and the magnet poles. Onthis bar there is a :rduce little error .since variations in the battery compensating coil, 99. Intypical ohmmeter cir- -Iproduce nosubstantial.variations in the current cuits the moving coil is connected as the cross @of the, moving coilingviewofthe operation of. the arm of an electrical bridgegandthecompensating cathode-follower stage 2| being substantially incoil is connected in series withthe source of v.oltsdependent of plate voltage,

age for the bridge. ,Thus, the current ,in the In- Figurel2 there is ,shown a revision of ,the m v n 00 1 w ll vary over a widerange-as from system.of-..Eigure 1 to convert ittoone of anull.

ii to 360. In this system the electrical instrument I05 is a double-throw relay having a pole 65 working between two contacts 66 as before shown in Figure 8. The circuit for this relay may comprise the bridge 30,'but alternatively the resistors 3|, 32 and 33 may be eliminated and the junction 34 may be joined directly to the junction 35 as whenever less sensitivity-less deflection per unit change of the condenser I'2is needed. The

I09 are bridged together the motor will turn in one direction and when the two I08 and H0 are bridged together the motor will turn in a reverse direction, the motor coming to standstill when these terminals are open. The pole terminal I08 is connected by a lead lead to the pole 65 of the relay I05 and. the control terminals I09 and H0 are connected by respective leads 39a and H011 to the respective contacts 65. Thus, when the relay closes in one direction the motor will turn one way and when the relay closes in the other direction the motor will turn the other way, the motor coming to standstill when the relay contacts open.

The motor has a shaft I I I which drives a shaft H2 at right angles thereto through worm gearing H3. The shaft II 2 has a drive connection, generally referred to as 211, to a pointer IM which registers with a scale H5. This scale may have any desired length up to a maximum of 360. Also, as through the drive connection II2a, the motor drives a movable arm II6a of a rheostat H6. This rheostat replaces the resistor 55 of the fixed voltage-divider circuit, this circuit being here referred to as I42). This system is arranged so that when the voltage on the condenser I2 and that across the rheostat i I6 are in a predetermined ratio, say a ratio of unity, the relay I05 is open with the pole 85 thereof being substantially midway between the two contacts 65. The operationof this system is as follows:

When the condenser I2 undergoes a predetermined change in capacity, the pole 65 is closed with one of the contacts $6 to start the motor running to shift the position of the pointer Ii l relative to the scale I I5 and concurrently to vary the rheostat H6. The relay, motor and drive connections of the motor to the'rheostat are poled so that this variation of the rheostat will bring the voltage thereacross substantially in balance with that across the condenser l2. When this condition of substantial balance is reached the relay I05 is again opened and the motor comes to a standstill. A change in the rheostat through its full range is accompanied by a change in the pointer II4 across the full length of the ,scale H5, and the scale H5 is so calibrated that the pointer will register with a division thereon corresponding'to the value of the condenser I2 or the quantity which the condenser represents, as when the voltages across the condenser and the rheostat are substantially in balance. Thus, there is obtained a long-scale measuring system, which is very desirable in aircraft fuel gauges because of the large quantity of fuel which the system must gauge. Also, since each reading is obtained under conditions of substantial balance, as to the "voltages taken from the two voltage-divider circults, errors due to temperature, etc. are substantially balanced out.

I have hereinabove particularly described my invention in terms of certain preferred embodiments, but these embodiments are intended to be illustrative and not necessarily limitative of my invention since the same are subject to changes and modifications without departure from the scope of my invention, which I endeavor to express according to the following claims.

I claim:

1. In an electrical system responsive to a variable quantity and including sources of alternating and direct current: the combination of means for providing a first AJ-C. voltage dependent both on said A.-C. source and said variable quantity; means connected to said A,-C. source for providing a second A.-C. voltage dependent on said A.-C. source and independent of said quantity; a pair of thermionic stages each including a cathode, grid and plate, means for feeding said A.-C. voltages to said grids respectively, means connecting said stages as cathode followers with the plates thereof being connected directly to said D.-C. source and with the grids thereof being biased relative to the respective cathode potentials to produce D.-C. voltages in the respective cathode circuits which vary substantially according to said respective A.-C. Voltages; a D.-C. type of electrical instrument having a movable element and coils for producing two relatively variable electromagnetic fields to deflect said element upon relative variation of two D.-C. currents; and a bridge-type load circuit between said cathodes and ground comprising a first branch serially including said coils and connected between the cathodes of said stages, a second branch serially including two impedances and connected also between said cathodes, and a third impedance connected between a point in said first branch and a point in said second branch.

2. The combination set forth in claim 1 includingmeans connected between said A.-C. and

*D.-C. sources for maintaining the voltage of said grids respectively, means connecting said stages as cathode followers with the grids thereof being biased relative to the respective cathode potentials to produce D.-C. voltages in the respective cathode circuits which vary substantially according to said respective A.-C. Voltages; a D.-C. type electrical instrument including a movable element and coils for deflecting said element upon variation of the D.-C. currents in said coils; a bridge circuit including said coils connected in series between said cathodes, and three resistors connected respectively from said cathodes and a point between said coils to a common junction;

and a circuit connection between said junction and ground of the system.

4. The combination set forth in claim 3 including electrical condensers connected across said coils for increasing effectively the current flow through the coils respectively. I

5. In an electrical system responsive to a variable quantity and including sources of A.-C. and D.-C. current: the combination of means for providing a first A.-C. voltage proportional both to that of said A.-C. source and to said variable quantity; means for providing a second A.-C. voltage proportional to that of said A.-C. source and independent of said quantity; a pair of thermionic devices each including a cathode, grid and plate; means feeding said A.-C. voltages to said grids respectively; means connecting said plates directly to said D.-C. source; means connected to D.-C. source for biasing said grids substantially in fixed proportion to the respective plate voltages to maintain said devices substantially at plate current cutoff; and a D.-C. type electrical instrument including a movable element and coils for producing two relatively variable electro-magnetic fields to deflect said element upon relative variation of two D.-C. currents, said coils being connected in the cathode circuits of said devices.

6. The combination set forth in'claim including a rectifier connected across said A.C. source and biased in fixed proportion to said D.-C. source for clipping peak portions of the positive half cycles of said A.-C. source whereby to maintain a fixed proportionality between the magnitude of said positive half cycles and the voltage supply to said plates.

'7. In an electrical system responsive to a variable quantity and including sources of A.-C. and D,-C. current: the combination of means for providing a first A.-C. voltage proportional both to that of said A.-C. source and to said variable quantity; means for providing a second A.C. voltage proportional to that of said A.-C. source and independent of said quantity; a pair of thermionic devices each including a cathode, grid and plate; grid-leak resistors connecting said grids to ground for the system; means feeding said A.-C. voltages to said grids respectively; means connecting said plates to said D.-C. source; a bleeder circuit connected from said D.-C. source to ground for the system; a D.-C. type instrument including a movable element and a pair of coils for deflecting said element in response to a relative variation of D.-C. current through said coils; and a bridge circuit including said coils and connected between said cathodes and a tap of said bleeder circuit, said tap being chosen to cause said thermionic devices to be biased substantially to plate current cutoff.

8. In an electrical system responsive to a variable quantity and including a source of alternating current: the combination of means for providing a first A.-C. voltage dependent both on said source and said variable quantity; means connected to said source for providing a second AC. voltage dependent on said source and independent of said quantity; a pair of thermionic devices each including a cathode, grid and plate connected as cathode followers, said A.-C. voltages being fed to said grids respectively; grid leak resistors connected between said grids and respective cathodes for biasing said grids substantially to the potential of the respective cathodes so that D.-C. voltage variations are produced in the cathode circuits of said devices according to the respective A.-C. voltages impressed on said grids; a D.-C. type electrical instrument including a movable element and coils for producing two relatively variable electro-magnetic fields to deflect said element upon relative variation of two D.-C. currents; and a circuit includ- 18 ing said coils and connected between said oath-- odes and ground for said system.

9. The combination set forth in claim 8 wherein said last-stated circuit is of a bridge type having a branch connected between said cathodes and serially including said coils, and including three resistors connected respectively from said cathodes and from the junction between said coils to said ground.

10. A rectifying system comprising a thermionic device having a cathode, grid and plate connected as a cathode follower; a load device connected between said cathode and ground of said system; means for supplying a potential to said plate; and means connected to said platesupplying means for biasing said grid in accordance with said plate potential substantially to plate current cutolf.

11. A rectifying system comprising a the'rmi onic device having a cathode, grid and plate connected as a cathode follower; means for supplying a D.C. potential to said plate; a grid-leak resistor connected between said grid and ground for said system; a bleeder circuit connected across said plate-supplying means and to said ground; and a load circuit for said device connected between said cathode and a point of said bleeder circuit to bias said grid substantially to plate current cutoff.

12. In an electrical system responsive to a variable quantity and including sources of direct and alternatin current: the combination of one circuit composed only of linear impedance elements one of which is variable and connected to said A.-C. source for providing a first A.-C. voltage proportional both to that of said A.-C. source and to said quantity; a second circuit connected to said A.-C. source for providing a second A.-C. voltage proportional to that of said A.-C. source and substantially independent of said quantity; a cathode-follower stage including a grid, cathode and plate connected to operate as a rectifier said plate being connected to said D.-C. source; means feeding said first A.-C. voltage to said grid; a D.-C. type electrical instrument including a movable element and two coils for deflecting said element upon a relative variation of D.-C. current in said coils; circuit means connected to said second circuit and including a rectifier for providing a D.-C. voltage proportional to said second A.-C. voltage; a circuit for feeding the rectified voltages from said cathode-follower stage and said rectifier to said coils, said circuit having a ground connection and being adapted to cause a relative change in the D.-C. current in said coils in response to a variation of said variable quantity; and means connected between said A.-C. and D.-C. sources for maintaining the effective voltages of said sources substantially in fixed proportion to each other.

13. In an electrical ratio system for measuring a variable electrical quantity, including A.-C. and D.-C. sources of potential: the combination of a first voltage-divider circuit connected across said A.-C. source and having an output terminal for providing a first A.-C. voltage, relative to ground of said system, which is proportional both to that of said source and to said variable quantity; a second voltage-divider circuit connected to said A.-C. source and having an output terminal for providing a second A.-C. voltage proportional to that of said source and substantially independent of said variable quantity; a pair of thermionic stages each including a cathode, grid and plate operated as a cathode follower; means connecting the plates oi said stages directly to said D.-C. source of potential; means connecting Said grids respectively to the output terminals of said divider circuits; means biasing said grids relative to the respective cathodes to cause D.-C. voltages to be produced in the respective cathode circuits which vary substantially according to the A.-C. voltages applied to the respective grids whereby said cathode-follower stages operate as rectifiers; a D.-C. ratio instrument comprising a movable indicating element and two associated coils for deflecting said element according to the ratio of the currents in said coils; and a load circuit comprising said coils and connected between the cathodes of said stages and ground, said load circuit being adapted to produce relatively varying currents in said coils according to variations in the relative potentials of the cathodes of said stages. 7

14. In an electrical system responsive to a variable quantity and including a source of alternating current: the combination of means for providing a first A.-C. voltage dependent both on said source and said variable quantity; means connected to said source for providing a second A.-C. voltage dependent on the source and independent of said quantity; a pair of thermionic devices each including a cathode, grid and plate operated as a cathode follower; grid leak resistors connected between said grids and ground of said system; means feeding said A.-0. voltages 20 to said grids respectively; a 11-0. ratio-type instrument including a movable element and two coils for deflectin said element in response to a variation in the ratio of the currents in said coils; circuits connecting said coils between said cathodes and a junction point in said system; and means between said junction and ground for supplying a D.-C. potential to said cathodes to bias said grids substantially to plate-current cutoff so that pulsating D.-C. currents are produced in the cathode circuits substantially in proportion to the respective A.-C. voltages applied to said grids. V

GEORGE H. FRITZINGER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Smith Dec. 16, 1947 

